Concrete in Australia Vol 39 No 2 29
Why Super Tees are a winner
Super Tees beat every competitor on virtually every aspect
I have seen, making them desirable and cost effective. e
Super Tee is a very efficient structural solution, giving lowest
material costs. Precasting off site gives flexible programming
and lower labour costs. e Super Tee has a very quick
and simple onsite construction method. It is very stable for
handling. It provides an immediate safe working platform on
erection, and has excellent durability performance and has
low maintenance for concrete. It can be detailed for simple
inspection and if necessary, bearing replacement. e Super
Tee may also be potentially reuseable.
Comment on calculations
for Super Tees
As an experienced designer, I find that many consultants
underestimate how difficult it is to design a bridge, and
Super Tee bridges for road or rail vehicles really do require
substantial knowledge and experience.
I will not catalogue the errors and misconceptions I have
come across, but all that early training on "city bridges" which
I was lucky enough to get is very hard to find for our newer
generation of engineers (and even some older ones it seems).
is is certainly a downside to the era of design and construct
and a problem which will get worse as the experience of people
like me dies out, unless the road authorities take some steps to
redress it.
To design Super Tees you really need to understand torsion
and how it relates to shear and the truss analogy. Unfortunately,
in that area the Australian Bridge Code AS5100.5 lets us down
badly. e most recent Concrete structures standard AS3600 is
a bit better but still has some serious drawbacks.
Sadly in my experience on Australian Standards committees,
the wheels turn slowly and often fail to get us where we want to
go. I urge anyone designing Super Tees, or any other concrete
box girder bridge, to use the Main Roads WA s Shear and Torsion
Rules. ese were developed by Main Roads WA working with
some external consultants including myself and have been
verified. ey are dove-tailed into the rules in the present
AS5100.5 and lead to a straightforward transparent design
process, which is clearly based on the truss analogy.
To be really efficient in design you should also make sure you
have a reliable section analysis program which can apply axial
tensile loads at the strength limit, such as RAPT.
Aesthetics of Super Tees
Bridges, even these lesser bridges, occupy large blocks of
our visual environment. ere will always be diehards --
probably at least partly including me -- who could never see
a precast beam type bridge in the same light as the simpler
minimalist lines of a box girder. Any attempt to make it more
aesthetically acceptable would in porcine terms be making a
silk purse out of a sow s ear, or in the blunter modern version,
putting lipstick on a pig -- at the end of the day it s still a pig.
However, if we can put aside the prejudices of people like me
who designed "city bridges", and notice that these Super Tees
are not a great rack of NAASRA I-girders, with Bondek forms
glinting between them, supported on hammerhead piers on a
couple of columns where you can still see the spiral form, you
can begin to see them in a different light.
ey have simple clean lines and reasonably large widely
spaced members, especially here in Western Australia where
we tend to make quite wide ones. e specifying authorities
should and do require some aesthetic guidelines to be followed.
In truth, apart from obvious rules like not varying the form
along the bridge, the superstructures virtually design themselves
aesthetically. It is in the piers, where a pair of bearings must be
provided along each beam line, where I believe we need to be
most sensitive.
Here in principle, I support what Main Roads WA does, but
must respectfully disagree with their prescriptively required
V-shaped pier. I have suggested that they (and other specifying
authorities) provide for piers as a provisional sum, so they allow
themselves input with the architect and builder, and control
over costs. With a well thought out pier shape these bridges
are at worst "unobtrusive" and at best really quite a neat, smart
solution.
Can Super Tees take over the world?
ere are all sorts of reasons why things happen which are not
necessarily the most apparently logical. In France, they love
grand, bold, aesthetic structures and they certainly have some
wonderful bridges. Perhaps there is such a culturally ingrained
aesthetic sense that it filters down to this type of bridge and
they will carry on much as we once thought we would with
our "city bridges".
In the UK, I have seen a generation of steel composite bridges
spawned by a ridiculous over reaction to one failure attributed
to poor grouting of post-tensioning cables and I suspect a very
powerful steel lobby. Someone should tell them that Super Tee
bridges use pretensioned strands -- would it make a difference
-- who knows? I couldn t see the steel producers thinking that it
should.
But I am a great believer that eventually market forces and
common sense will prevail -- and yes -- Super Tees will take over
the world, unless someone thinks of an even better idea.